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Date: Sat, 15 Nov 2014 00:54:24 +0400 From: Andrey Ryabinin <ryabinin.a.a@...il.com> To: Joe Perches <joe@...ches.com> Cc: Andrey Ryabinin <a.ryabinin@...sung.com>, LKML <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, Sasha Levin <sasha.levin@...cle.com>, Randy Dunlap <rdunlap@...radead.org>, Rasmus Villemoes <linux@...musvillemoes.dk>, Jonathan Corbet <corbet@....net>, Michal Marek <mmarek@...e.cz>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Yury Gribov <y.gribov@...sung.com>, Dmitry Vyukov <dvyukov@...gle.com>, Konstantin Khlebnikov <koct9i@...il.com>, "x86@...nel.org" <x86@...nel.org>, linux-doc@...r.kernel.org, linux-kbuild@...r.kernel.org Subject: Re: [PATCH v2 1/2] kernel: printk: specify alignment for struct printk_log 2014-11-14 20:22 GMT+03:00 Joe Perches <joe@...ches.com>: > On Fri, 2014-11-14 at 15:50 +0300, Andrey Ryabinin wrote: >> On architectures that have support for efficient unaligned access >> struct printk_log has 4-byte alignment. >> Specify alignment attribute in type declaration. >> >> The whole point of this patch is to fix deadlock which happening >> when UBSan detects unaligned access in printk() thus UBSan recursively >> calls printk() with logbuf_lock held by top printk() call. > [] >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > [] >> @@ -223,7 +223,11 @@ struct printk_log { >> u8 facility; /* syslog facility */ >> u8 flags:5; /* internal record flags */ >> u8 level:3; /* syslog level */ >> -}; >> +} >> +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS >> +__packed __aligned(4) >> +#endif > > Why is adding __packed useful or __aligned(4) useful? > On x86_64, from compiler's point of view, 'struct printk_log' should be 8-byte aligned. But it stored in log buffer only with 4-byte alignment (LOG_ALIGN define). This inconsistency makes UBSAN unhappy. UBSAN uses printk to report errors, thus any access to 4-byte aligned 'struct printk_log' causing recursive printk() call from ubsan handler. And if logbuf_lock was taken we end up with deadlock. Specifying alignment removes this inconsistency. Now compiler knows that 'printk_log' actually aligned on 4, and it makes UBSAN happy and silent. Attribute 'aligned' without attribute 'packed' can only increase alignment. So __packed is used here because we need to decrease alignment from 8 to 4. > The struct is naturally aligned on u64 and should be Not, always. On 32-bits it will be 4-bytes aligned, on 64-bits - 8-bytes aligned > the size of 2 u64s. > > struct printk_log { > u64 ts_nsec; /* timestamp in nanoseconds */ > u16 len; /* length of entire record */ > u16 text_len; /* length of text buffer */ > u16 dict_len; /* length of dictionary buffer */ > u8 facility; /* syslog facility */ > u8 flags:5; /* internal record flags */ > u8 level:3; /* syslog level */ > }; > > Is there any case when it's not sizeof(u64) * 2? > I think no. As I said __packed used for decreasing alignment only. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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